Role of the giant elastic protein titin in the Frank-Starling mechanism of the heart

Increased ventricular volume enhances the systolic performance, a phenomenon known as Frank-Starling's law of the heart. At its basis is the ability of cardiac muscle to produce increased active force in response to increased muscle length. Although numerous studies have been conducted to elucidate the molecular basis of length-dependent activation, the mechanism remains elusive. The giant protein titin (also known as connectin) is the third filament system in the sarcomere and is responsible for most passive stiffness of striated muscle in the physiological sarcomere length range. The force generated by titin is usually seen as passive and independent of active force generation. Recent findings, however, suggest that titin-based passive force modulates actin-myosin interaction, resulting in greater active force in response to stretch. In this short review, we discuss the molecular mechanisms of length-dependent activation, focusing on the possible role of titin in its regulation.     

Do people match surface reflectance fundamentally differently than they match emitted light?

We compared matches between colours that were both presented on a computer monitor or both as pieces of paper, with matching the colour of a piece of paper with a colour presented on a computer monitor and vice versa. Performance was specifically poor when setting an image on a computer monitor to match the colour of a piece of paper. This cannot be due to any of the individual judgments because subjects readily selected a matching piece of paper to match another piece of paper and set the image on the monitor to match another image on a monitor. We propose that matching the light reaching the eye and matching surface reflectance are fundamentally different judgments and that subjects can sometimes but not always choose which to match.     

Protein kinase A phosphorylates titin's cardiac-specific N2B domain and reduces passive tension in rat cardiac myocytes

beta-Adrenergic stimulation of cardiac muscle activates protein kinase A (PKA), which is known to phosphorylate proteins on the thin and thick filaments of the sarcomere. Cardiac muscle sarcomeres contain a third filament system composed of titin, and here we demonstrate that titin is also phosphorylated by the beta-adrenergic pathway. Titin phosphorylation was observed after beta-receptor stimulation of intact cardiac myocytes and incubation of skinned cardiac myocytes with PKA. Mechanical experiments with isolated myocytes revealed that PKA significantly reduces passive tension. In vitro phosphorylation of recombinant titin fragments and immunoelectron microscopy suggest that PKA targets a subdomain of the elastic segment of titin, referred to as the N2B spring element. The N2B spring element is expressed only in cardiac titins, in which it plays an important role in determining the level of passive tension. Because titin-based passive tension is a determinant of diastolic function, these results suggest that titin phosphorylation may modulate cardiac function in vivo.     

Structure-function relations of the giant elastic protein titin in striated and smooth muscle cells

The striated muscle sarcomere contains, in addition to thin and thick filaments, a third myofilament comprised of titin. The extensible region of titin spans the I-band region of the sarcomere and develops passive force in stretched sarcomeres. This force positions the A-bands in the middle of the sarcomere, maintains sarcomere length homogeneity and, importantly, is responsible for myocardial passive tension that determines diastolic filling. Recent work suggests that smooth muscle expresses a truncated titin isoform with a short extensible region that is predicted to develop high passive force levels. Several mechanisms for tuning the titin-based passive tension have been discovered that involve alternative splicing as well as posttranslational modification, mechanisms that are at play both during normal muscle function as well as during disease.     

Knockout of Lmod2 results in shorter thin filaments followed by dilated cardiomyopathy and juvenile lethality

Leiomodin 2 (Lmod2) is an actin-binding protein that has been implicated in the regulation of striated muscle thin filament assembly; its physiological function has yet to be studied. We found that knockout of Lmod2 in mice results in abnormally short thin filaments in the heart. We also discovered that Lmod2 functions to elongate thin filaments by promoting actin assembly and dynamics at thin filament pointed ends. Lmod2-KO mice die as juveniles with hearts displaying contractile dysfunction and ventricular chamber enlargement consistent with dilated cardiomyopathy. Lmod2-null cardiomyocytes produce less contractile force than wild type when plated on micropillar arrays. Introduction of GFP-Lmod2 via adeno-associated viral transduction elongates thin filaments and rescues structural and functional defects observed in Lmod2-KO mice, extending their lifespan to adulthood. Thus, to our knowledge, Lmod2 is the first identified mammalian protein that functions to elongate actin filaments in the heart; it is essential for cardiac thin filaments to reach a mature length and is required for efficient contractile force and proper heart function during development.Striated muscle cells contain arrays of protein filaments assembled into contractile units that are nearly crystalline in structure. Efficient contraction at the molecular level is predicated upon accurate overlap of actin-containing thin and myosin-containing thick filaments. Therefore, proper control of filament assembly is absolutely critical.In striated muscle it is currently thought that the thin-filament pointed end capping protein tropomodulin (Tmod) is the predominant regulator of thin filament length, with Tmod1 being the sole isoform expressed in cardiomyocytes (1). Extensive in vitro work has revealed that Tmod1 uses two actin- and two tropomyosin-binding sites to associate with the end of the thin filament and to prevent addition or loss of actin monomers, thereby controlling length of the thin filament (2–7). Tmod1 is essential for life; Tmod1-KO mice are embryonic lethal because of cardiac defects (8–11).Identification of additional but structurally different members of the Tmod family of proteins, the leiomodins (Lmods), raises the possibility that thin filament lengths are not regulated solely by Tmod at thin filament pointed ends (12). Although there are three Lmod genes (Lmod1–3), Lmod2 and 3 are expressed in striated muscle with Lmod2 being the predominant isoform in cardiac muscle and Lmod3 the predominant isoform in skeletal muscle (12–16). The Lmods share ∼40% sequence identity at the protein level with the Tmods but do not contain a recognizable second tropomyosin-binding domain and have an additional C-terminal extension that includes a proline-rich region and an actin-binding Wiskott–Aldrich syndrome protein homology 2 (WH2) domain (12, 17). Lmod2 has been proposed to be the long-sought muscle actin filament nucleator because it robustly nucleates actin filament formation in vitro (because of its three actin-binding sites) and is reportedly required for proper sarcomere assembly in cultured cardiomyocytes (17). Like Tmod1, Lmod2 assembly at the pointed end of the thin filament requires association with tropomyosin; however unlike Tmod1, Lmod2 assembly also is dependent on contractility and the availability of polymerizable actin (18). Although part of the Tmod family of proteins, Lmod2 does not demonstrate actin filament-capping activity, and its overexpression displaces Tmod1; it is not known if this displacement is a direct or indirect effect (13). Nevertheless, Lmod2 overexpression results in the elongation of thin filaments in cells in culture (13). Limited data regarding the function of Lmod2 suggest it could play an important role in sarcomeric actin assembly, but the physiological function of Lmod2 has yet to be studied.Here we show that Lmod2 functions as an actin filament elongation factor in the heart. Our search for the mechanism by which Lmod2 functions revealed that Lmod2 promotes actin assembly and dynamics at the pointed end of the thin filament, is not necessary for myofibrillogenesis, but is required for thin filaments to attain a mature length. Our results also indicate that Lmod2 is essential for normal heart function and suggest that dysregulation of the thin filament length is causative for dilated cardiomyopathy (DCM).     

Physiological functions of the giant elastic protein titin in mammalian striated muscle

The striated muscle sarcomere contains the third filament comprising the giant elastic protein titin, in addition to thick and thin filaments. Titin is the primary source of nonactomyosin-based passive force in both skeletal and cardiac muscles, within the physiological sarcomere length range. Titin's force repositions the thick filaments in the center of the sarcomere after contraction or stretch and thus maintains sarcomere length and structural integrity. In the heart, titin determines myocardial wall stiffness, thereby regulating ventricular filling. Recent studies have revealed the mechanisms involved in the fine tuning of titin-based passive force via alternative splicing or posttranslational modification. It has also been discovered that titin performs roles that go beyond passive force generation, such as a regulation of the Frank-Starling mechanism of the heart. In this review, we discuss how titin regulates passive and active properties of striated muscle during normal muscle function and during disease.